W3C
validator.w3.org
The W3C Markup Validation Service
... This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc. If you wish to validate specific content such as RSS/Atom feeds or CSS stylesheets, MobileOK content, or to find broken links, there are other validators and tools available.
W3 Nu Html Checker
This tool is an ongoing experiment in better HTML checking, and its behavior remains subject to change · Show sourceoutlineimage reporterrors & warnings onlycheck error pagesUser-Agent Accept-Language
W3C Feed Validation Service, for Atom and RSS
This is the W3C Feed Validation Service, a free service that checks the syntax of Atom or RSS feeds.
w3 Validator
This Link Checker looks for issues in links, anchors and referenced objects in a Web page, CSS style sheet, or recursively on a whole Web site. For best results, it is recommended to first ensure that the documents checked use Valid (X)HTML Markup and CSS. The Link Checker is part of the W3C's ...
W3C
jigsaw.w3.org › css-validator
The W3C CSS Validation Service
Note: If you want to validate your CSS style sheet embedded in an (X)HTML document, you should first check that the (X)HTML you use is valid.
Videos
06:19
How to use W3C HTML and CSS code validators - YouTube
How To Validate Your HTML5 Code Using An Online Validation Service ...
Validating Code with the W3C Validator
05:55
Using the W3C validator to check HTML - YouTube
16:17
How To Validate HTML Code – What You Need To Know - YouTube
01:49
Using the W3C Markup Validation Service to check your HTML - YouTube
W3C Validator alternatives for broken HTML?
Which of w3 validators do you use? What doctype is set on your documents? Give some examples of "valid things" that show errors in validator? More on reddit.com
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May 9, 2025html - Javascript Validation w3c - Stack Overflow
Paste your code at http://vali... "Validate Document fragment, XHTML 1.0". ... Sign up to request clarification or add additional context in comments. ... Thanks for the answer! Unfortunately, it didn't seem to do it.. Should I be reducing the More on stackoverflow.com
What is the W3C VALIDATOR?
The w3c validator is one of the many HTML validators that checks your HTML markup to see if you are following the rules for constructing proper HTML. What is Markup Validation? Most pages on the World Wide Web are written in computer languages (such as HTML) that allow Web authors to structure text, add multimedia content, and specify what appearance, or style, the result should have. As for every language, these have their own grammar, vocabulary and syntax, and every document written with these computer languages are supposed to follow these rules. The (X)HTML languages, for all versions up to XHTML 1.1, are using machine-readable grammars called DTDs, a mechanism inherited from SGML. However, Just as texts in a natural language can include spelling or grammar errors, documents using Markup languages may (for various reasons) not be following these rules. The process of verifying whether a document actually follows the rules for the language(s) it uses is called validation, and the tool used for that is a validator. A document that passes this process with success is called valid. With these concepts in mind, we can define "markup validation" as the process of checking a Web document against the grammar (generally a DTD) it claims to be using. Web browsers are actually very tolerant of mistakes and try to render the HTML as best it can no matter how broken it is. For example, you can create a page missing several closing tags and it will still render something. So just because the HTML renders in the browser doesn't mean all is well. There are no rules as to how invalid HTML should be handled, which is why every browser handles these mistakes in different ways. So it's important to always try and serve up valid HTML. More on reddit.com
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January 7, 2023Validator.w3.org
I am so confused and frustrated now. I was revisiting my first completed project (tribute page) and I have it on git. I watched the video on HTML/CSS from freecodecamp the most recent one to help me in areas I felt I was struggling with. I learned about using the Validator.w3.org site to check ... More on forum.freecodecamp.org
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July 3, 2022W3Schools
w3schools.com › js › js_validation.asp
JavaScript Form Validation
HTML form validation can be done by JavaScript.
GitHub
github.com › w3c › markup-validator
GitHub - w3c/markup-validator · GitHub
The tool consists of a perl-based CGI script that uses DTD to verify the validity of HTML3, HTML4 and XHTML documents; it also incorporates by reference the NU Validator used to validate HTML5 / HTML LS documents. W3C Software License and Notice.
Starred by 137 users
Forked by 32 users
Languages HTML 79.1% | Perl 13.3% | Python 2.2% | CSS 2.1% | Clean 1.3% | JavaScript 0.9%
Reddit
reddit.com › r/webdev › w3c validator alternatives for broken html?
r/webdev on Reddit: W3C Validator alternatives for broken HTML?
May 9, 2025 -
I've always used the W3C Validator to help find broken HTML elements, but I'm finding it's becoming quite outdated and throwing errors for things that are now valid.
Are there any better alternatives to finding broken HTML elements?
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Which of w3 validators do you use? What doctype is set on your documents? Give some examples of "valid things" that show errors in validator?
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The main W3C validator has gone horrifically out of date. WHATWG recommends using the Nu HTML Checker from W3C instead: https://validator.w3.org/nu/ Nothing is more reliable than putting the HTML into a variety of web browsers and checking the console output, though.
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Use CDATA sections. The < has a special meaning for the parser.
Copy<script>
//<![CDATA[
...JavaScript code..
//]]></script>
The validator has detected a <, and attempts to parse a new tag <q.length. When the semicolon is spotted, the parser don't know how to handle it, and throws an error. By using CDATA, you effectively say "Anything inside this section should be interpreted as plain-text, and not be parsed.
Paste your code at http://validator.w3.org/check (validate by direct input), using the following settings: "Validate Document fragment, XHTML 1.0".
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If you move your javascript functions into an external file, and then link to that file from your page head section, then the validator doesn't have to worry about them.
e.g.
Copy<script src="functions.js" type="text/javascript"></script>
This also allows you to reuse functions across multiple pages without repeating the code.
A further benefit is that the user's browser will cache the js file the first time it encounters it, so on subsequent pages the javascript functions file will be retrieved from cache, rather than being downloaded with the page.
W3C
w3.org › developers › tools
Validators and tools | Developers | W3C
This page lists the validators and web developer tools.
npm
npmjs.com › package › w3c-html-validator
w3c-html-validator - npm
January 21, 2026 - Check the markup validity of HTML files using the W3C validator. Latest version: 2.2.0, last published: 2 months ago. Start using w3c-html-validator in your project by running `npm i w3c-html-validator`. There are 4 other projects in the npm registry using w3c-html-validator.
» npm install w3c-html-validator
Published Jan 21, 2026
Version 2.2.0
Author Thomas Davis
Reddit
reddit.com › r/learnprogramming › what is the w3c validator?
r/learnprogramming on Reddit: What is the W3C VALIDATOR?
January 7, 2023 -
Hello this is willijkm speaking. actually i read a book about html and in a chapter of this book speak about W3C VALIDATOR but i don't quiet understand it, Can you help me? Thanks.
freeCodeCamp
forum.freecodecamp.org › html-css
Validator.w3.org - HTML-CSS - The freeCodeCamp Forum
July 3, 2022 - I am so confused and frustrated now. I was revisiting my first completed project (tribute page) and I have it on git. I watched the video on HTML/CSS from freecodecamp the most recent one to help me in areas I felt I was struggling with. I learned about using the Validator.w3.org site to check your code.
FreeFormatter
freeformatter.com › html-validator.html
Free Online HTML Validator - FreeFormatter.com
Validates HTML files for compliance against the W3C standards and performs linting to assess code quality against best practices.
Search Engine Journal
searchenginejournal.com › sej › seo
W3C Validator: What It Is & Why It Matters For SEO
February 14, 2022 - Either way, these errors will show up in the W3C validator. The above are just some examples of errors that you may see when you’re validating your page. Unfortunately, the list goes on and on – as does the time spent trying to fix these problems! You may find more specific errors that apply to your site. They may include errors that reference “type attribute used in tag.” · This refers to some tags like JavaScript declaration tags, such as the following: <script type=”text/javascript”>.
SitePoint
sitepoint.com › html & css
Validation errors due to Javascript? - HTML & CSS - SitePoint Forums | Web Development & Design Community
July 30, 2011 - I recently hired a freelancer to validate all the code for my Wordpress e-commerce website (http://www.projectdisobey.com/disobeyclothing) He told me that he had finished validating everything, but when I tested each page out using the validation service at http://validator.w3.org/, I came across a whole load of validation errors (http://www.projectdisobey.com/disobeyclothing/wp-content/uploads/2011/10/ValidationErrors.jpg).
MDN Web Docs
developer.mozilla.org › en-US › docs › Learn_web_development › Extensions › Forms › Form_validation
Client-side form validation - Learn web development | MDN
JavaScript, including solutions using maxlength, can be used to provide this. Note: Length constraints are never reported if the value is set programmatically. They are only reported for user-provided input. For numeric fields, including <input type="number"> and the various date input types, the min and max attributes can be used to provide a range of valid values.
npm
npmjs.com › package › html-validator
html-validator - npm
(async () => { const validator = require('html-validator') const options = { url: 'http://url-to-validate.com', validator: 'WHATWG', format: 'text' } try { const result = await validator(options) console.log(result) } catch (error) { console.error(error) } })() or run the w3c validator locally with docker $ docker run -d -p 8888:8888 validator/validator and
» npm install html-validator
Published May 14, 2022
Version 6.0.1
Author Geir Gåsodden
Repository https://github.com/zrrrzzt/html-validator
Reddit
reddit.com › r/webdev › anyone regularly check / address w3c validation of html?
r/webdev on Reddit: Anyone regularly check / address W3C validation of HTML?
August 23, 2024 -
Because I have not. It always felt like a throwback to the early days of HTML, and with the (necessary) forgiving nature of browsers, it rarely reaches the front of my mind. Chasing accessibility, style updates, color contrast, page weight, security testing, end-to-end testing, 100 other things,... it just never rises to the top. Some of the findings feel pedantic (only accepted text values of a `<date>` tag), some feel worthwhile (duplicate IDs on a page).
Curious about others' experiences.
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Invalid HTML is a source of potential accessibility issues. For example, if you have an inout inside label tag and the. wrap that input into div, some assistive technologies will start treat it like it's in a group (fieldset) which is confusing for users. Also, sometimes browsers "fix" markup not in a way you want, even if it looks okay. Then there is important feature in the validator - showing outline of your page structure, which is critical for accessibility. Overall, checking your pages once in a while (or even automating those checks) can pay out if you are doing accessible web.
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Applying WCAG AA accessibility requirements makes any website/app very Usable even for a normal (not with the disability) user. By learning to design and develop with the goal of achieving at least AA compliance, the final product becomes instantly more Usable and user-friendly. Some would say that is too much to give up for the 'design'. But that is an another story to discus ..
Validator
about.validator.nu
About Validator.nu
JavaScript bits, the favicon and a lot of bug reports were contributed by Simon Pieters. The schemas for RELAX NG and XSLT were written by James Clark. The principal author of the schema for DocBook is Norman Walsh. The SVG schemas come from the W3C.
Quora
quora.com › What-is-the-difference-between-an-HTML-validation-tool-and-a-W3C-validator
What is the difference between an HTML validation tool and a W3C validator? - Quora
Answer: An HTML validator parses and tokenizes each line of code to check for syntax errors such as elements missing delimeters or other required parameters. Syntax errors such as open tags, misspellings, forgotten quotation marks, colons or ...
Chrome Web Store
chromewebstore.google.com › detail › w3c-markup-validation-ser › kfjemflfclknofbklcnbnpjelkfhcomk
W3C Markup Validation Service for CSR pages - Chrome Web Store
A tool for validating the currently rendered html with the Markup Validation Service provided by the W3C. It provides the ability to extract the HTML of the current page and populate values into Validate by Direct Input in the Markup Validation ...
Site24x7
site24x7.com › tools › javascript-validator.html
Javascript Validator | Validate Online - Site24x7 Tools
... Microsoft Exchange server monitoring. Troubleshoot problems relating to mail servers. ... JavaScript validation ensures data integrity, security, and user experience by verifying inputs on the client-side before submission, reducing errors and improving user satisfaction.